T unit-containing branched silicone resins are highly heat-resistant and have an excellent capacity for film-formation, and for these reasons are used as electrical insulating materials, heat-resistant paints and coatings, protective coatings, physical property-improvers for curable organic resins, precursors for copolymers (in combination with organic resin monomers), and so forth. Among these silicone resins, the epoxy-containing DT silicone resins and epoxy-containing DTQ silicone resins (D=difunctional siloxane unit, Q=tetrafunctional siloxane unit) are used as physical property-improvers for curable epoxy resins, and these resins have been proposed in large number (Japanese Patent Application Laid Open Number Sho 56-145942 and Japanese Patent Publication Numbers Sho 58-53655 and Sho 62-27095).
The epoxy-containing silicone resins proposed in Japanese Patent Application Laid Open Number Sho 56-145942 and Japanese Patent Publication Numbers Sho 58-53655 and Sho 62-27095 are prepared either by the cohydrolysis and condensation of epoxy-containing trialkoxysilane and diorganodialkoxysilane or by the base-catalyzed equilibration polymerization of cyclic diorganosiloxane and epoxy-containing trialkoxysilane. However, due to the use of these particular preparative methods, the resulting epoxy-containing silicone resins have broad molecular weight distributions and do not exhibit a softening point or a distinct glass-transition temperature. Moreover, they have poor stabilities because their molecules contain large quantities of silanol groups or silicon-bonded alkoxy groups. These drawbacks serve to limit their applications.
Moreover, the methods proposed in Japanese Patent Application Laid Open Number Sho 56-145942 and Japanese Patent Publication Numbers Sho 58-53655 and Sho 62-27095 for the preparation of epoxy-containing silicone resins are incapable of providing control over the molecular weight, softening point, and glass-transition temperature of the epoxy-containing silicone resin product. In consequence thereof, these methods are incapable of the highly reproducible synthesis of epoxy-containing silicone resins having a freely selectable molecular weight, softening point, and distinct glass-transition temperature in the range of -90.degree. C. to 150.degree. C.
Bodying is a known technology that consists of the base-catalyzed depletion of the silanol groups in a silicone resin prepared by the hydrolysis and condensation of organoalkoxysilane. However, the molecular weight of the silicone resin afforded by bodying is simply increased, while control of the molecular weight, softening point, and glass-transition temperature is not possible. Furthermore, the molecular weight, softening point, and glass-transition temperature of the resulting epoxy-containing silicone resin are substantially altered as a function of the degree of bodying, and this basically eliminates any possibility for the highly reproducible synthesis of epoxy-containing silicone resin that has a freely selectable molecular weight, softening point, and distinct glass-transition temperature in the range of -90.degree. C. to 150.degree. C.
Curable resin compositions cure to yield resins that have excellent electrical properties (dielectric properties, volume resistivity, insulation breakdown strength, and so forth) and excellent mechanical properties (flexural strength, compressive strength, impact strength, and so forth), and for this reason are used in the widest range of applications.
However, the resins afforded by the cure of such curable resin compositions are stiff and inflexible and they exhibit a large cure shrinkage. As a result, when these resins are used as sealants for electrical/electronic elements or as coatings or adhesives, large forces are applied to the substrate, adherend, or electrical/electronic element, and the resin is subject to cracking and/or rupture, resin-sealed electrical/electronic elements may be destroyed, and gaps may be produced between the resin and adherend or between the resin and electrical/electronic element.
In addition, curable resin compositions cure to give resins whose thermal expansion coefficient is larger than that of electrical/electronic elements. When an electrical/electronic element sealed with such a resin is repeatedly subjected to thermal shock, the reliability of the element is reduced due to cracking in the resin, destruction of the resin-sealed element, and the generation of gaps between the resin and the element.
In order to improve the flexibility, moisture resistance, and heat-shock resistance of these cured resins, curable resin compositions composed of curable resin and organopolysiloxane have been proposed in large number. Curable resin compositions of this type are exemplified by the curable resin composition proposed in Japanese Patent Application Laid Open Sho 56-136816 and the previously cited Japanese Patent Application Laid Open Number Sho 56-145942. The former is composed of epoxy resin, (epoxy+methoxy)-containing organopolysiloxane, curing agent, and inorganic filler. The latter is composed of an epoxy resin+silanol-containing organopolysiloxane mixture, epoxy-functional organopolysiloxane, inorganic filler, and curing catalyst.
The epoxy-functional organopolysiloxanes used in the curable resin compositions proposed in Japanese Patent Application Laid Open Numbers Sho 56-136816 and Sho 56-145942 are, however, prepared by the cohydrolysis of epoxy-functional alkoxysilane and another type of alkoxysilane, and curable resin compositions prepared by the addition of such epoxy-functional organopolysiloxanes yield cured resins that do not exhibit a satisfactory flexibility, moisture resistance, or heat-shock resistance.
The present invention takes as an object the introduction of epoxy-containing silicone resin that contains the T unit and that has a distinct glass-transition temperature in the range of -90.degree. C. to 150.degree. C. An additional object of the present invention is the introduction of a method for the preparation of epoxy-containing silicone resin that is capable of a highly reproducible synthesis of epoxy-containing silicone resin that contains the T unit and that has a freely selectable molecular weight, softening point, and distinct glass-transition temperature in the range of -90.degree. C. to 150.degree. C.
The present invention also takes as its object the introduction of a curable resin composition, based on the above silicone resin, that has an excellent fluidity prior to curing and that cures to form resin that is very flexible, highly moisture resistant, and strongly resistant to heat shock.